The commonly-owned patent application cited above teaches modification of a coal tar pitch by nitrile butadiene rubber. Coal tar, both refined and pitch, is known to have many inherently advantageous properties with respect to its performance in roofing and waterproofing applications, including chemical resistance, ultraviolet resistance, and ozone resistance. However, it is also known that coal tar has physical limitations to its use. For example, the low softening point of coal tar limits it to low or no slope applications. The brittleness of coal tar at room temperatures or below causes it to crack, delaminate and show poor cohesive strength. Also, coal tar is used with some reservation due to its high level of volatile emissions upon heating in kettle applications. Further, when coal tar compositions are used to fill cracks and joints between adjacent concrete slabs on pavement surfaces or to form films for waterproofing membranes on concrete slabs, it is essential that the composition adhere well to the concrete substrate. Tars and pitches, in their native form, have the unsatisfactory property of becoming brittle and cracking, as well as pulling away from the concrete. This can result in water penetration and the subsequent breaking and cracking of the concrete from freeze/thaw cycles. While the cold flow and self-healing properties of coal tar will correct some of these problems at elevated temperatures and over time, the loss of sealing capability can compromise the waterproofing qualities. While the previous work cited has shown that the flow resistance of the modified coal tar may be increased at elevated temperatures and the softening point may be increased, the particular compositions taught exhibit no marked improvement in T.sub.g, the well-known transition temperature for polymeric materials. Above the T.sub.g of a given material, it will possess a rubbery or semiliquid quality, allowing flexibility, while below the T.sub.g, the material will act in a glassy or brittle manner and tear or fracture rather than flex. It is a desired goal to reduce the T.sub.g of a coal tar composition so that the desired flexibility is available even when ambient temperatures drop.
Other modification of coal tar pitches has used styrene-based copolymers. These polymers are thermoplastic elastomers, such as polystyrene-polybutadiene-polystyrene ("SBS") or polystyrene-polyisoprene-polystyrene ("SIS") block copolymers. An unfortunate requirement of these modifiers is the high percentage of polymer needed to achieve the elastomeric properties and improve high temperature flow resistance. They also require process oils to be added to increase the compatibility of the polymers with the coal tar blends. The high viscosity exhibited makes these materials unsuitable as an "applied in place" waterproofing material. Other known crack sealant formulations based on poly(vinyl chloride) ("PVC") and plasticizers to lower the T.sub.g are not suitable in roofing applications, since they are too viscous. Also, these materials tend to have a very short pot life.
Combining coal tars with NBR latexes in aqueous emulsions for use with appropriate fillers is used commonly for applying a waterproofing seal to asphalt pavements.
For these and other reasons, it is desirable to modify a coal tar to produce a material which softens at a higher temperature than unmodified coal tar while lowering the glass transition point, to increase the range of flexibility and cohesive strength.